I recently contacted Demon Carburetion about the flow rate of my 650 Speed Demon.
I was told the stock jets for the primaries is 70 and the secondaries is 78, but they could not tell me the combined full fuel flow rates @ 6psi at WOT.
They said it varies with each engine. I do not understand?
If I maintain 6 psi of fuel pressure why can't they use the K factor for the jets and calculate the flow rate. What does the engine have to do with it?

Please help explain this to me! :confused:

 

The amount of fuel that flows through the jets is based upon the amount of airflow through the carb, which is determined largely by the amount of air that is flowing through the engine and different engines flow different amounts of air. On a carburated engine, the purpose of higher pump pressure is to refill the float bowls when fuel demand is high, not to force fuel through the jets.

 

Vetterodder,
Thanks, that makes a lot of sense to me now that the air flow is in the equation.
Is there any way to calculate air flow without doing a chass. Dyno run?
I have a problem with my engine at around 5500 rpm, it just seems to die. I really do not know the problem and I am searching for clues.
Thanks again, Bud

 

Fuel pressure fills the bowls. Air going through the venturi sucks fuel through the jets from the bowls. Therefore fuel flow into the engine is based on air flow, and at a metered rate by the jets.

Hope that clears it up.

 

Bowls are vented. Not pressure fed. If that helps. You just have to have something that keeps up.. Most motors never do WOT for more than 10-15 seconds at a time. So the need for high pumps is a mith. Fuel logs and a return keeps you from having vapor lock. That is in any carb.

 

Just for info... I order a 750 race demon... they are supposed to have airflow in the 900-925 cfm range. Thats a lot of air!

Cory

 

I was sure the 750 flowed 750cfm...which would explain the name. :)

 

Are you familiar with high-end carbs? Judging from your ET... I'd guess not. It's very common for a "worked" carb to outflow a stock rating. That's why it costs significantly more than a regular old Holley.

Cory


[Modified by CHarris85Vette, 8:04 AM 11/5/2001]

 

I always wondered what the scientific reason was for a 750 CFM required bigger jets than the 750 cfm double pumper holley to get the same air fuel ratio. I thought it must be the boosters in some way. More CFM than advertised would account for the difference. I have since changed to non removable vent. 825 Race Demon which isn't as crisp on the low end. So I may change back to my Speed Demon with my little 383 ci.

 

Holy Cow! Maybe one tech guy didn't bone me! LOL :) I was warned against the 825 cfm model for my application by a Barry Grant Tech. He said I'd loose the low end that I needed to get out of the corners quicker. He said run the 750 Race Demon unless I went larger than a 383. I even asked his opinion on a 350 build that would turn 7500 rpm (my next engine). He still said stay with the 750cfm model. I guess the 825 flows closer to a 1000 cfm and has larger throttle blades so it loses low end sensitivity. From your comments, I'd guess he was right on.
Wouldn't it be nice to have the model with the removeable sleeves? I thought hard about kicking the extra cash for it but decided not to since the 750 should meet all my needs.

Cory

 

CHarris - I could have traded out my 750 or 825 for his removable. When my buddy sold his race car. It was the typical thing where he got 30 cents on a the dollar for a BBC 10.90 class Gas Chevelle. So it wouldn't have matter what it had on when he sold it as long as it ran good. He had the 850's inserts in a 472 ci for consistancy.

I can't iron out the below 2300 rpm bog. I just haven't played with it enough. You would need bigger than my .048 squirters and you would need to put in a faster squirter cam. The power really hits hard right after the bog because my 3000 stall takes over and it will put you into a four wheel drift really fast. So you just have to learn throttle control. No putting to instant WOT, you just ease into it. The 750 I had pretty well figured out and it can be back on in ten to 15 minutes.

 

In the newest issue ( I think it is anyway) of Chevy High Performance, there is a "carb shoot-out" article. They use several holley carbs (I think it was a 650 vac, 650 mech, 750 vac) and several Demon carbs (same type spread pretty much). Problem is that they used two different cars and ran all the holleys on one and all the demons on the other. Not much of a "shoot-out" at all. It does give some interesting info as to what you might gain when you swap from vacuum secondaries to mechanical. They typically showed "little" gains .10 - .15 et gains but those are pretty big gains for just slapping on a different carb. They also clearly didn't have enough time to optimize tuning either so more gains are likely available. I wish they would have run the carbs on the same car just for comparison purposes. I'd really like to know just how much better the demon is.
To get to the point... CHP did provide some jetting info on all the carbs straight out of the box. The Demon is indeed jetted larger in every example as compared to the equivalent RATED (what they sell the carb as- i.e. 650cfm) holley carb.
I'm anxious to get my car started and sorted out.

Cory